Perpendicular transport in isotropic magnetic turbulence. The role of stochastic drifts

Abstract: In cosmic ray propagation through turbulent magnetic fields, transport perpendicular to the mean field plays an important role in various astrophysical environments. Numerical simulations have shown that parallel and perpendicular diffusion coefficients exhibit different energy scalings, causing their ratio to increase with energy, but a comprehensive theoretical explanation remains elusive. Here, we investigate perpendicular diffusion using test particle simulations in synthetic isotropic turbulence spanning 6-7 orders of magnitude in wavenumber, together with a theoretical extension of nonlinear guiding center theory that incorporates stochastic drifts. Our approach combines field line random walk and random guiding center drifts to describe perpendicular transport. We find that parallel and perpendicular diffusion coefficients exhibit the same energy scaling only for $r_L \lesssim 10^{-2} L_c$, where $r_L$ is the particle Larmor radius and $L_c$ is the turbulence coherence length. At higher energies, perpendicular diffusion develops a stronger energy dependence, consistently across different turbulence levels. Our theoretical model suggests that this behavior arises from stochastic drifts induced by turbulent magnetic field components parallel to the mean field.